This invention relates to a wet friction member having a friction characteristic that changes with the sliding direction, and to a wet frictional engagement apparatus such as a wet friction clutch for a transmission using such a wet friction member.
So far, no wet friction member has been known having a friction characteristic that changes with the sliding direction.
In a hydraulically operated transmission, when speed of a vehicle is changed, for example, from third speed to second speed by a downshift operation in which oil is discharged from the third-speed hydraulic clutch that establishes the third-speed gear train and at the same time oil is charged to the second-speed hydraulic clutch that establishes the second-speed gear train, the third-speed hydraulic clutch can be disengaged slightly earlier to put the transmission in neutral temporarily so as to cause the engine to react to slightly increasing its speed of revolution, so that the difference in number of revolutions between the input side and the output side of the second-speed hydraulic clutch is reduced for smooth engagement of the second-speed hydraulic clutch, thus minimizing gear shift shocks.
However, if the transmission remains in neutral for too long, the engine revolution speed increases to an excessively high level. To prevent this, charging and discharging of oil to and from the second-speed hydraulic clutch and third-speed hydraulic clutch is controlled so that the second-speed hydraulic clutch may become engaged slightly before the difference in number of revolutions between the input side and output side of the second-speed hydraulic clutch becomes zero. As a result, when engagement of the second-speed hydraulic clutch is being performed, the revolution speed of the input side of that clutch is forcibly increased from a lower level to that of the output side, so that all the load for increasing rotations of the all the rotating members ranging from the clutch to the engine should be taken on by the output side of the second-speed hydraulic clutch. This results in a temporary sharp drop in the driving torque and therefore an unsmooth shifting of gears.
A transmission has been known which has a one-way clutch built into its first-speed gear train to allow overrotation of the output side of the first-speed gear train. In such a transmission when a downshift operation is performed to change speed from second speed to first speed, the disengagement of the second-speed clutch causes the engine to race to increase its speed of revolution When the revolutions of the input side of the one-way clutch is increased with such engine racing so much as to almost exceed that of the output side, the one way clutch becomes engaged to establish the first-speed gear train. Therefore, the output side of the transmission does not take on the load that causes the revolution speed of the input side to increase, thus providing a smooth gear shifting.
As mentioned above, with a gear train provided with a one-way clutch, lowering of the driving torque that can occur in the course of gear shifting is prevented to assure a smooth shifting of gear. However, the gear train provided with a one-way clutch cannot transmit a reverse driving torque from the drive wheel so that there arises a problem that engine braking does not work at all. Because of this problem, the one-way clutch cannot be incorporated in the second-speed and third-speed gear trains that are operated in the medium and high speed ranges.
In the above instance, it may be possible to provide, in parallel with a one-way clutch in the second-and third-speed gear trains, a hypothetical engagement device which is directly connectable to both the input and output sides of the clutch so that, when a throttle opening is reduced to its lowest degree in order to effect engine braking the hypothetical engagement device would be activated to directly couple the input side and the output side of the one-way clutch for transmitting the reverse driving torque from the output side to the input side. This, however, would result in many problems such as an increase in the size and weight of the transmission due to addition of the engagement device and a complicated structure of the hydraulic control device.
If a hydraulic clutch is equipped with a clutch plate such as a wet friction member whose friction characteristic changes with the sliding direction so that the clutch may come to more easily slip under a condition in which the output side member of the clutch is overrotating relative to the input side member thereof, it is possible to minimize gear shift shocks even without use of a one-way clutch.